In the silver salt diffusion process, an imagewise exposed silver halide emulsion layer is positioned in intimate contact with a receptor element and developed in the presence of a silver halide solvent such that the unexposed silver halide diffuses into the receptor element where it is developed to provide an image. Development nuclei or catalyst, contained in the receptor element, cause the diffusing silver halide to be reduced to metallic silver, thereby forming an image on the receptor element surface.
More specifically, in the silver halide diffusion transfer process a developer composition is used which contains a developing agent, e.g., hydroquinone, a solubilizing agent for silver halide, e.g., the thiosulfate ion and, preferably, an antifoggant, e.g., benzotriazole. When an imagewise exposed plate is contacted with the developer composition, the light exposed silver halide grains are reduced to silver metal in normal fashion. The unexposed grains dissolve in the developer via formation of soluble silver complexes, such as the complexes of silver thiosulfate. When the soluble silver complex contacts a development nuclei or catalyst, typically contained in a receptor layer or element, the silver is reduced to a metallic deposit. This deposit can then form the ink receptive image areas of a lithographic plate.
There are many variations on this theme, such as: some or all of the developer may be initially incorporated within the plate structure; the plate may contain either a positive or negative silver halide emulsion; the development nuclei or catalyst-containing layer may be on a separate support or receptor element and brought into intimate contact with the silver halide emulsion after being wet with developer composition, following which the two separate supports are peeled apart; a receiving layer containing development nuclei or catalyst can be contained as an intermediate layer between the initial support and the silver halide emulsion, such as is taught in U.S. Pat. No. 4,204,868; or the receptor layer can be contained on top of the silver halide emulsion layer, as is taught in U.S. Pat. Nos. 3,728,114 and 4,160,670.
While the construction having a receptor layer between the emulsion and support can provide an excellent lithographic plate, the entire emulsion layer must be removed in this case to provide functionality as a lithographic plate. The silver contained in the developing medium must then be recovered in a separate operation. Conversely, with the receptor layer overlying the emulsion, this recovery step can be eliminated.
In the case where the receptor layer overlies the emulsion, it is apparent that the receptor layer is extremely important to the functionality of the resultant structure as a lithographic plate. If the image on the plate surface is not highly ink receptive, the resultant copies during the initiation of a press run will not be fully inked, resulting in paper waste. Obviously, if the silver image is buried too deeply in the receptor layer, the plate may not in fact print at all, even if rubbed with a so-called "starter solution."
Belgian Pat. Nos. 709,193; 709,194; 716,778; and 720,695 describe a printing plate having a very thin receiving layer to increase the capability for ink acceptance by the imaged silver, and containing a very low content of a binding agent, taught to be a protective colloid. However, even with a very low content of protective colloid, it has been found to be necessary to use the aforementioned starting treatment to provide the necessary ink receptivity to image areas.
U.S. Pat. No. 3,547,641 provides for an emulsion layer having a transparent, hardened, non-removable, water permeable colloid layer containing a nucleating agent or catalyst for silver deposition by the diffusion transfer method, the layer being impregnated with a water-miscible organic solvent containing a water-soluble sulfide, such as sodium sulfide. The problem with this approach is that the soluble sulfide, during development, penetrates the individual layers as well as remaining on the surface, thereby providing only partial effect as a receiving layer; furthermore, the powerful fogging effects of sulfide on silver halide emulsions are well-known and are undesirable in this instance.
U.S. Pat. No. 3,728,114 provides for an photolithographic structure having a receiving layer which is extremely thin, taught to be less than 30 Angstroms in order to provide for appropriate silver image presentation at the plate surface. In this instance the development nuclei or catalyst are in the form of sols, i.e., heavy metals or their sulfides in sol form so as to provide a dispersion and distribution of the nuclei as individual particles. The nuclei can be applied with up to about 30 percent by weight of a high molecular weight hydrophilic compound (based on the weight of nuclei).
U.S. Pat. No. 4,160,670, taught to be an improvement over this patent, indicates that the lithographic element of this patent has disadvantages in that tolerance limits are narrow for various commercial inks, printing characteristics vary with the lapse of time before or after the making of the plate, and toning is liable to occur in long run printing.
This improvement U.S. Pat. No. 4,160,670, teaches the use of polyfunctional water soluble polymers to maintain ink receptivity of the deposited silver and improve toning during long press runs. The polymer has one structural unit containing a functional group having an affinity for development nuclei in sol form and a second structural unit capable of imparting hydrophilicity to the polymer. However, low molecular weight polymers of the type described are so water soluble little benefit is provided and high molecular weight polymers, while providing a clean plate background, can tend to cause poor rollup at the start of the press run, i.e., wherein the image areas will not optimally accept ink.
Surprisingly, we have now found that a high molecular weight hydrophilic polymeric material can be added to the receiving layer, the material having the surprising effect of not only increasing the hydrophilic properties of plate, but also improving the ink receptive properties of the silver surface image. The polymeric material found to have such utility is a polyaldehyde.